FLASH memories are rapidly finding numerous new fields of application. Their success is primarily due to the relatively small dimension of the unit cell, which is about equal to that of a traditional EPROM cell, coupled to the ability of being electrically erased. These characteristics ideally place FLASH memories between traditional EPROM memories (less expensive) and the EEPROM memories (much more expensive) from which they differ be being electrically erasable although the erasing may be carried out exclusively on the whole memory matrix, while an EEPROM memory may be erased byte by byte. On the other hand, a typical EEPROM unit cell is about four times larger than a FLASH cell.
It is easily understood why the FLASH memories have conquered large sectors of the market where it is important to have nonvolatile, large-capacity memories which may be electrically erased but whose cost is relatively moderate.
One of the industrial sectors which is particularly interested to this type of memories is the automotive sector where on-board reprogramming of the memories is an indispensable requirement, e.g., for modifying the controlling parameters of a motor from time to time, or for recording the operating history of a motor between services, and the like.
Intrinsically a FLASH memory may develop specific programming disturbances, which make this type of memory very delicate from the point of view of the design of the circuitry for managing the memory matrix. The erratic programming phenomena known as "soft-programming" and a similar erratic erasing or "soft-erasing" to which the cells may frequently undergo, create remarkable programming problems for this type of memories from the pointer of view of these recurrent physical error mechanisms.
The organization in a matrix of rows and columns of the cells of a FLASH memory is schematically depicted in FIG. 1. Within row 1, there is a cell (A) which is to be programmed to store either the high state, "1," or a low state, "0." A cell (B) has already been programmed to store the high state, "1." A cell (C) is a virgin cell, that is, it has not been programmed. By soft-programming is meant an eventually unwanted programming which a virgin cell (C) may undergo when other cells (A) disposed on the same row of the virgin cell (C) are being programmed. If the total number of cells arranged on the same row is very large and if all the cells of the row are programmed in succession, the repeated electrical stresses induced by high programming voltages (12 V on the gate of the cell to be programmed) to which the virgin cell (C) is repeatedly subjected may lead to an unwanted programming thereof.
With reference to FIG. 1, by soft-erasing is meant the possibility that a programmed cell (B,D) may improperly be brought back to a condition of a virgin cell because of the repeated electrical stresses induced by high erasing voltages (12.5 V on the gate for the (B) cell, 6.5 V on the drain of the (D) cell) to which the programmed cell is subjected when the programming of a cell which is on the same row or column of said B and D cells is carried out. If the number of cells, rows and columns of the matrix is large, this error mechanism becomes even more relevant than that due to soft-programming because it has a more relevant overall effect than soft-programming.
In both cases, the physical mechanism through which soft-programming or soft-erasing takes place is the so-called Fowler-Nordheim current effect.
It has also been observed that the unit cells of the memory more subject to soft-programming are the cells of the reference columns (reference hit lines) of the matrix, which provide a reference of the threshold and current levels for reading by means of the sense-amplifier of the managing circuitry of the memory, the data stored therein. Moreover the repeated electrical stresses to which the reference cells are subjected have a negative influence on the stability in time of the relative threshold and current levels provided by the cells, thus often causing problems of reliability of the reading circuitry of these memories.